"High-performance car bumpers" as used in the instant disclosure and claims are those car bumpers where the increase in theoretical stiffness relative to a normal bumper, with other conditions such as car overhang, fastening means, resistant section (moment of inertia), elastic modulus, and so forth, being the same, varies within the range of from 2.5 to 6 times as great, with a simultaneous decrease in overall tension conditions.
Suitable shock absorbers make possible the absorption of energy by yielding to the shocks in order to avoid adverse effects to the chassis of a car.
Car bumpers exhibit relatively poor performances in Europe where impact speed values are up to 4 km/hour. The performance threshold is normally limited by economic constraints and short shifts allowed for by the bumper before interfering with the frame or other functional parts of the car, such as the radiator. Within these limits, damage to the bumper is also normally allowed.
In North America greater displacements are permitted due to the bumper protruding further from the car than European bumpers. Performances corresponding to impact speeds of 8 km/hour against a front barrier are possible. Normally, under such conditions, shock absorbers are used, e.g., shock absorbers of hydraulic type, in rigid-beam systems where the rigid beam can be protected by energy absorbing materials, such as flexible foam materials. Metal bumpers are included within the category of supporting-beam bumpers. To be able to operate within their range of reversibility (absence of yield or of permanent deformation), metal bumpers require front energy absorbers, of foamed material types with great front displacements, or absorbers installed on brackets by which the bumper structure is anchored to the car, such as hydraulic shock absorbers.
Usually, the maximum value of the force which can be transmitted to the chassis of the car is not reached within the operating range of bumpers, front bumpers in particular. Therefrom, the allowed shifts being the same, the actual impact speed is decreased considerably as compared to the maximum impact speed which can be theoretically achieved.
With the aim of achieving meaningful increase in performance in terms of impact speed, the following remarks can be made regarding energy: if the value of total force which can be transmitted to the structure of the car is fixed each time and other conditions being the same,
the optimum situation of energy absorption for purposes of reducing the displacements consists of obtaining a rectangular shape of the graph of the force values vs. the displacement values (the variables), i.e., the energy absorption should take place already because the first impact moment, at the maximum allowed force, which is a preset constant. The availability of an infinite stiffness of the system which yields under a constant controlled force is assumed. The system operates within its plastic range. PA1 The maximum force being the same, the energy stored by a system in the elastic range is represented by a triangular graph of the force values vs. the displacement values (the variables). In this situation, where the surface area (i.e., the energy) is the same and the limits of resistance are typical for the materials used, the displacements are greater than in the previous situation. PA1 Elastic systems protected by shock absorbers or displaying elasto-plastic phenomena, show lower maximum force values than in the purely elastic situation, where the relevant displacements are consequently greater.